This invention relates to combustion apparatus and, more particularly, to means for providing effective film cooling of combustion chambers. For convenience of illustration and discussion, the invention will be described in connection with a jet engine of the gas turbine type. However, it will be appreciated that the structure is suitable for any high temperature application which requires effective film cooling.
Aircraft engines presently in operational use and those under development for future application are designed to operate at extremely high temperatures. Combustors associated with such engines must not only be compatible with the high temperature environment but must also perform efficiently for extended periods of time before removal for repair and maintenance. Since the life of a combustor liner is directly affected by the temperature at which it operates, efficient and reliable means for cooling the combustor and lowering its operating temperature must be provided.
State-of-the-art cooling means for combustor chambers have provided a moving film of cooling air between the inner surface of the combustor liner and the hot gas stream. The film of cooling air prevents the hot gas stream from contacting the combustor liner and transferring heat thereto. Generally, the protective film is introduced into the combustion chamber from a plenum surrounding the combustor.
It is essential for optimum effectiveness that the film of cooling air forming the protective boundary between the combustor liner and the hot gas stream be continuous. Furthermore, the film must be introduced at a velocity and direction preselected to avoid intermixing with the hot gases. Generally, the cooling film must consist of a uniform layer of cooling air having a uniform exit velocity around the cooled periphery of the liner.
State-of-the-art cooling devices have attempted to achieve effective cooling of the combustor liner by a variety of means. The earliest devices simply introduced cooling air through a series of apertures in an upstream portion of the liner into a lipped annular pocket wherein the streams of fluid from the individual apertures were permitted to coalesce to form a uniform boundary. The cooling air then emerged from the lipped pocket into the combustion chamber along the inner surface of the liner. These early devices were unacceptable for at least two reasons. First, the apertures were located such that they admitted cooling air having a high dynamic pressure head and hence a high total pressure head incompatible with the formation of an efficient boundary layer film. Secondly, the high velocity of the cooling air passing through the apertures required a long, extended lip to permit the fluid streams to coalesce before emergence into the combustor chamber. The long lip is subject to thermal stresses which cause warpage and buckling of the lip. Later prior art devices have introduced dimpled lips in an attempt to solve the aforementioned warpage problem. Combustors with dimpled lips however proved to have limited life due to hot spots created in wakes in the boundary layer caused by the dimples and due to rapid crack propagation inherent in a dimpled design.
Still later prior art devices provided improvements in combustor liner cooling by introducing means to eliminate the dynamic pressure head from the cooling air and baffle means for diffusing the cooling fluid streams prior to their emergence into the combustor chamber. These prior art devices have not been successful in providing a uniform film of cooling air as boundary layer protection to the combustor liner.